The present invention relates to methods and apparatus for detecting microorganisms in blood culture samples, where the specimen and a culture medium are introduced into a sealable container or vial and are exposed to conditions enabling a variety of metabolic, physical, and chemical changes to take place in the presence of microorganisms within a sample.
Usually, presence of biologically active agents such as bacteria in a patient's body fluid, especially blood, is determined using blood culture vials. A small quantity of blood is injected through the enclosing rubber septum into the sterile vial containing a culture medium. The vial is incubated at 37.degree. C. and monitored for bacterial growth.
Common visual inspection involves monitoring the turbidity or eventual color changes of the liquid suspension. Known instrumental methods detect changes in the carbon dioxide content of the culture bottles, which is a metabolic by-product of the bacterial growth. Monitoring the carbon dioxide content can be accomplished by methods well established in the art, such as radiochemical, or infrared absorption at a carbon dioxide spectral line. These methods require invasive procedures which result in the well-known problem of cross-contamination between different vials. For purposes of this application, the term invasive implies that the confines of the vial must be entered in order to determine if microorganisms are present, e.g., a probe is inserted into a sealed vial. In addition, due to the fact that head space gas (i.e. the gas in the vial above the sample) must be removed for analysis, there is a requirement for adding head space gas to the vials after each measurement.
It has also been proposed to detect bacterial growth in sealable containers by monitoring positive and/or negative pressure changes (e.g., U.S. Pat. No. 4,152,213--Ahnell, the disclosure of which is incorporated by reference). Any temperature change within the vial head space, however, generates a pressure change which is not related to biological activities. Therefore, a head space temperature measurement is required in order to distinguish between biological and temperature-generated pressure changes. Non-invasive head space temperature monitoring, however, is a difficult problem. Additionally, some microorganisms can produce high pressure values, while others produce relatively low or negligible changes. Thus, any pressure sensor used must be sensitive enough to allow detection of small changes in pressure while also being capable of safely measuring high pressure values. These two requirements are often mutually exclusive depending on the type of pressure sensor technology used. As a result, some microorganisms may be difficult to detect by pressure monitoring using available technology.
Recently, non-invasive methods have been developed involving chemical sensors disposed inside the vial. These sensors respond to changes in the carbon dioxide concentration by changing their color or by changing their fluorescence intensity. (See, e.g. Thorpe et al. "BacT/Alert: An Automated Colorimetric Microbial Detection System", J. Clin. Microbiol., July 1990, pp. 1608-12, and U.S. Pat. No. 4,945,060--Turner, et al., the disclosure of which is incorporated by reference). These techniques are based on light intensity measurements and require spectral filtering in the excitation and/or emission channels. With these techniques, errors can occur if the light source, the photodetector, the filters, or the sensor show aging effects over time which would vary the intensity response. Additionally, certain species of microorganisms are weak carbon dioxide producers. Therefore, it is possible that sensors based on carbon dioxide monitoring may fail to detect certain microorganisms.
It has been suggested in U.S. patent application Ser. No. 07/874,324, the disclosure of which is incorporated by reference, to take advantage of changes in the hemoglobin absorption as well as changes in light scattering caused by bacterial activities in blood cultures. This detection principle is based on scattered photon migration (SPM) in a highly absorbing and highly scattering medium whereby, no diffusion effect into a sensing membrane takes place, and no corresponding delay time is observed. However, if the SPM detection method is used alone, one loses the benefit of the carbon dioxide detection method, which is able to detect most of the microorganism species.